1. Field of the Invention
The invention relates to a control system for an internal combustion engine equipped with a catalytic converter for purifying exhaust gas from the engine, and, more particularly, to a fuel combustion control system for providing combustion stability when an ignition timing is retarded in order to rise the temperature of a catalyst of the catalytic converter during a cold start of the engine.
2. Description of the Related Art
Typically, there have been known various types of catalytic converters for purifying exhaust gas from an engine. Such an catalytic converter incorporates a three way catalyst to purify or significantly lower emission levels of unburnt hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx) and the like which can pose a health problem for the nation if uncontrolled. The three way catalyst is hard to present desired catalytic conversion efficiency if it is at lower temperatures and is, however, activated to present catalytic conversion efficiency when heated higher than a specific temperature sufficiently.
In this type of engine control system, it has been known to accelerate a rise in catalyst temperature by significantly retarding an ignition timing after top dead center to present desired catalytic conversion efficiency which is dictated by a temperature of engine cooling water while the catalyst has not yet attained an activated condition necessary. One of the engine control systems of this type is known from, for example, Japanese Unexamined Patent Publication No. 8-232645. This engine control system causes a large retard of an ignition timing to provide a large exhaust heat loss with an effect of rising an exhaust gas temperature, as a result of which the catalyst is quickly heated and activated sufficiently. Further, in order to prevent fuel combustion from being made unstable due to a retard of ignition timing, the engine control system is designed and adapted to improve ignitability and combustibility of an air-fuel mixture by generating a swirl of intake air in the combustion chamber, increasing a speed of an intake air stream or rising ignition energy.
The prior art engine control system has a necessity to have much scope for controlling an engine, so that the engine does not exceed a limit on necessary combustion stability. This imposes a restraint on retarding an ignition timing to its limit. In view of this, it is desired to improve activation of the catalyst by rising the temperature of exhaust gas.
As a practical matter, fuel is varies in quality such as heaviness to some extent, so that it is the possibility that the engine is deprived of combustion stability if a fuel of inferior ignitability and combustibility is used. When a limit to combustion stability is exceeded, the engine encounters an increase in vibration and a sharp increase in harmful emission level.